Comparison · Sedimentary Rocks
Shale vs Siltstone
Shale and Siltstone are easy to confuse because both are sedimentary rocks. Siltstone is harder (Mohs 3.5) than Shale (Mohs 2–3), so Siltstone will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Shale and Siltstone
- Siltstone is harder (Mohs 3.5) than Shale (Mohs 2–3), so Siltstone will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
- Luster is different: Shale is dull to earthy (slightly sub-vitreous if micaceous) while Siltstone is dull.
- Hold a thin edge to the light: Shale is Opaque, Siltstone is opaque.
- They overlap in gray, brown, red, and green, but Shale also comes in black.
Shale vs Siltstone properties
| Property | Shale | Siltstone |
|---|---|---|
| Type | Sedimentary Rocks | Sedimentary Rocks |
| Mohs hardness | 2–3 | 3.5 |
| Luster | Dull to earthy (slightly sub-vitreous if micaceous) | dull |
| Streak | — | Variable; often light gray to brown |
| Transparency | Opaque | opaque |
| Specific gravity | 2.4–2.8 | 2.45 |
| Crystal system | — | varies |
| Common colors | Gray, Black, Brown, Red, Greenish gray | Gray, Tan, Brown, Red, Green |
| Rarity | common | common |
| Typical specimen value | $1–$75+ | $1–$75+ |
Which is more valuable?
Typical specimens of Shale and Siltstone trade in a similar range, around $10.
About Shale
Shale is a fine-grained sedimentary rock formed from compacted clay and silt-sized particles, distinguished by its characteristic fissility, meaning it splits easily into thin, flat layers parallel to its original bedding planes. It accumulates in low-energy environments such as lake bottoms, river deltas, and offshore marine basins where fine sediment can settle undisturbed. Shale is one of the most abundant sedimentary rocks on Earth, making up a substantial portion of the sedimentary rock record worldwide.
About Siltstone
Siltstone is a fine-grained sedimentary rock made mostly of silt-sized particles. It feels smoother than sandstone but grittier than claystone or shale.
Shale vs Siltstone FAQ
Is Shale the same as Siltstone?
No. Both are sedimentary rocks, but they are distinct rock types with different hardness, streak or luster. Siltstone is harder (Mohs 3.5) than Shale (Mohs 2–3), so Siltstone will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not.
How can you tell Shale from Siltstone?
Siltstone is harder (Mohs 3.5) than Shale (Mohs 2–3), so Siltstone will scratch Shale and not the other way round. A copper coin (Mohs 3.5) scratches it, but a fingernail does not. Luster is different: Shale is dull to earthy (slightly sub-vitreous if micaceous) while Siltstone is dull. Hold a thin edge to the light: Shale is Opaque, Siltstone is opaque.
Which is harder, Shale or Siltstone?
Siltstone is harder: Mohs 3.5 against Mohs 2–3 for Shale.
Which is more valuable, Shale or Siltstone?
Typical specimens of Shale and Siltstone trade in a similar range, around $10.
